Differential hardening question

[Norman B]

I saw this is another thread placed in this forum:

a "traditionally" made japanese sword has a glass hard edge with a dead soft back. This is done by using a very shallow hardening steel and coating the back with a layer of clay so that during the quench the back misses the nose and stays soft.---it is a differential harden rather than a differential temper. it also gives the Hamon.

I knew that the clay coating created what was called a differential temper which results in the 'hamon' line along the blade. I did not know that this resulted in the back of the blade being 'dead soft'. I would have thought that the whole sword would have to be hardened, the cutting edge being the hardest, in order to withstand the rigors of use. Typically a knife is hardened throughout, while a sword is bigger, the width is still about the same as many knives. So, why would having a dead soft back NOT weaken the blade?

[Jeff Ellis]

Originally posted by Norman B
I saw this is another thread placed in this forum:



I knew that the clay coating created what was called a differential temper which results in the 'hamon' line along the blade. I did not know that this resulted in the back of the blade being 'dead soft'. I would have thought that the whole sword would have to be hardened, the cutting edge being the hardest, in order to withstand the rigors of use. Typically a knife is hardened throughout, while a sword is bigger, the width is still about the same as many knives. So, why would having a dead soft back NOT weaken the blade?

it's not "dead" soft, its just not as hard as the edge.. the edge is like.. 62 or 63 rc and the spine is 50 or so.. i might be a bit off on those numbers..

anyway, the softer back absorbed most of the shock the blade took. theres a load of information on the forums really.. differential tempering isnt the proper name, differential hardening is.

it came from the application of clay to the back of the blade in a certain way that kept it insulated and hot when the edge was cooled in the water..

dont quote me, i have a minimal amount of knowledge on the actual process..

the search function should pull up a plethora of information...

[Josh Powell]

It weakens the blade for certain functions, strengthens it for others.

In general, a differentially heat treated blade is less likely to snap in half, but is more likely to bend if it is exposed to lateral forces.

josh

[Ty N.]

There's always some confusion between 'selective hardening' and 'differential tempering'. Fortunately, there are plenty of old threads on this topic, so I would recommend using the Search function to explore this further.

[Patrick Hastings]

Originally posted by Jeff Ellis
it's not "dead" soft, its just not as hard as the edge.. the edge is like.. 62 or 63 rc and the spine is 50 or so.. i might be a bit off on those numbers..

For traditionally made blades the spine is more like Rc22-25. Modern performance swords like HC bainite blades might be more like Rc50 on the spine. You can get todays higher carbon blades up rather high without making them fully martensetic. The Japanese really had no middle ground it was either full hard or dead soft. "Tempering" (Stress-relieving) was abit of a mystery to the Japanese for a very long time.
A sword that is full Hard all the way through will snap easily. A sword that is dead soft all the way through will not hold an edge. When your understanding of metal and swords only give you the two choices the best solution for a sword is to combine the two and that gives you the creation of the differentially hardened blade. The blade is still susceptible to bending perhaps more so than other cultures in similar era. This weakness was partially compensated for by the skill of the user. Bad technique can bend a traditional Japanese sword and or chip the edge. It is not as bad as it sounds. The lower carbon range used and the shallow hardening nature of Tahamagane also helped with taming the brittleness of the edge. Thicker spines help keep the swords from bending as well.

[Jeff Ellis]

Originally posted by Patrick Hastings
For traditionally made blades the spine is more like Rc22-25. Modern performance swords like HC bainite blades might be more like Rc50 on the spine. You can get todays higher carbon blades up rather high without making them fully martensetic. The Japanese really had no middle ground it was either full hard or dead soft. "Tempering" (Stress-relieving) was abit of a mystery to the Japanese for a very long time.
A sword that is full Hard all the way through will snap easily. A sword that is dead soft all the way through will not hold an edge. When your understanding of metal and swords only give you the two choices the best solution for a sword is to combine the two and that gives you the creation of the differentially hardened blade. The blade is still susceptible to bending perhaps more so than other cultures in similar era. This weakness was partially compensated for by the skill of the user. Bad technique can bend a traditional Japanese sword and or chip the edge. It is not as bad as it sounds. The lower carbon range used and the shallow hardening nature of Tahamagane also helped with taming the brittleness of the edge. Thicker spines help keep the swords from bending as well.

d'oh, forgot that bit...

i knew i was off anyways, thanks patrick.

[Maurice Cate]

Not to mention that 62-63 RC would make a fairly brittle cutting edge that would be prone to chipping against harder targets like a wood dowel. From what I understand 58-59 RC for the cutting edge is as hard as you want to go.


Maurice

[Kevin R. Cashen]

Originally posted by Josh Powell
It weakens the blade for certain functions, strengthens it for others.

In general, a differentially heat treated blade is less likely to snap in half, but is more likely to bend if it is exposed to lateral forces.

josh

Josh your are right on with the concept, just a little hiccup in terminology, but don't sweat it at all, since about 80% of the population seem to slip on this one. The Pearlitic spine can "toughen" a blade but not strenghten. Pearlite never adds strength (the ability to resist deformation), unless the prior microstructure happened to be spheroidal cementite, or courser pearlite, with even less strenght than the current pearlite.

The edge, in this case, has the ultimate strength. It will resist deformation to the point that it will fracture before it will allow it.

[Ty N.]

I think Howard Clark rates the edges on his L6 at around HRC 57. I'm not sure about the spine. I've heard of other blademakers who produce three different levels of hardness in the same blade, although I don't know how this is executed.

[Andrew W. Priestley]

Originally posted by Ty N.
I think Howard Clark rates the edges on his L6 at around HRC 57. I'm not sure about the spine. I've heard of other blademakers who produce three different levels of hardness in the same blade, although I don't know how this is executed.

Ashi in the Hamon (little lines of softer steel than run in toward the edge at regularish intervals) are very pretty to look at but also serve the purpose of arresting cracks and chips that may form. They will only go to the next ashi line before they snap off in the softer steel, thus preventing the potential loss of the whole edge. Pretty neat concept. Less necessary in monosteel blades than in old forge folded blades.

[Carlo Giuseppe Tacchini]

Originally posted by Andrew W. Priestley
Ashi in the Hamon (little lines of softer steel than run in toward the edge at regularish intervals) are very pretty to look at but also serve the purpose of arresting cracks and chips that may form.

A thing frequently forgot in this forum. Not only shape follows
function but activities of the steel too...
With good peace of all the people thinking activities on japanese blades are merely estetics. They became highly regarded
because of they are useful too..

[Josh Powell]

Thanks for the correction, Kevin.

Makes perfect sense.

josh

Originally posted by Kevin R. Cashen
Josh your are right on with the concept, just a little hiccup in terminology, but don't sweat it at all, since about 80% of the population seem to slip on this one. The Pearlitic spine can "toughen" a blade but not strenghten. Pearlite never adds strength (the ability to resist deformation), unless the prior microstructure happened to be spheroidal cementite, or courser pearlite, with even less strenght than the current pearlite.

The edge, in this case, has the ultimate strength. It will resist deformation to the point that it will fracture before it will allow it.

[John Lundemo]

Were you to coat say a kat blade with clay and scrape clay off edge like traditional japanes styled blades and then Marquench in hot oil, you can acheive a 58 60 edge with a spine of 49 50 rockwell, if you used a 1075 or 1085 steel. This will create a differential hardness and with a good polish you can see the hamon. It would not be as vivid as the traditional water quench.